Field of the Invention
The present invention relates to a light shielding member provided to a lens in a lens array.
Description of Related Art
Aspherical optical elements (mirrors, lenses, etc.) are commonly used to reduce the size of optical systems included in optical devices, such as cameras, optical drives, semiconductor exposure devices, and the like. Efficient production of high-quality aspherical optical elements requires measurement technology of high accuracy to easily evaluate the shape of aspherical optical elements.
A well-known example of such measurement technology is a measurement instrument that uses a Shack Hartmann wavefront sensor (SHWS) having a microlens array (MLA) and an optical detector. When light cast onto an object to be tested such as an aspherical lens or the like is reflected, that light is propagated as modulated light with a wavefront having the shape of the object to be tested. The modulated light with a wavefront having the shape of the object to be tested is referred to, hereinafter, as “test light”. Detecting the test light using the SHWS enables wavefront measurement, and consequently the shape of the object to be tested can be measured from the wavefront measurement data.
When the test light enters the MLA of the SHWS, multiple spots are formed on the imaging surface of the optical detector. The spots are imaged, and the positions of the spots are each detected. The incident angle of the beam entering the microlenses is calculated from the detected spot positions, and data of the wavefront of the test light can be calculated from the incident angle distribution of the beam of test light.
In order to increase (improve) the measurement resolution of the SHWS, the pitch of the microlenses making up the MLA needs to be narrowed. However, light incident on the microlenses generates diffracted light owing to the perimeter edge portions of the microlenses. Accordingly, narrowing the pitch results in the electric field of the diffracted light generated at adjacent microlenses to interfere with the electric field of the spots, creating interference fringes. This distorts the spot images, and the wavefront measurement precision deteriorates. Accordingly, diffracted light nearby the spots that adjacent microlenses form needs to be suppressed, in order to realize both high resolution and high precision measurement of the SHWS. Japanese Patent Application Laid-Open No. 2013-2819 discloses SHWS technology where aperture members are added to each microlens of the MLA. In this case, the light transmittance of the aperture members decreases in stages toward the perimeter edge portions, starting at 100%, and then decreasing to 60%, and then 30%.
However, the aperture members described in Japanese Patent Application Laid-Open No. 2013-2819 has not been capable to sufficiently suppress diffracted light from interfering with spots generated by adjacent lenses. As a result, distortion of light spots formed by the lenses has not been able to be sufficiently suppressed, and sufficient wavefront measurement precision has yet not been obtained.
Therefore, it continues to be desirable to effectively suppress distortion in light spots formed by diffracted light of adjacent lenses of an MLA of a SHWS.